The present invention relates to rotary regenerative air preheaters having sector plates dividing the air preheater into a gas sector and one or more air sectors and having radial seals which cooperate with the sector plates to reduce the leakage between the air preheater sectors. More particularly, the invention relates to control systems for sector plate drive means for adjusting the position of the sector plates relative to the radial seals.
A rotary regenerative air preheater transfers sensible heat from the flue gas leaving a boiler to the entering combustion air through regenerative heat transfer surface in a rotor which turns continuously through the gas and air streams. The rotor, which is packed with the heat transfer surface, is divided into compartments by a number of radially extending plates referred to as diaphragms. These compartments are adapted to hold modular baskets in which the heat transfer surface is contained.
The air preheater is divided into a flue gas side or sector and one or more combustion air sides or sectors by sector plates. Flexible radial seals on the rotor, usually mounted on the top and bottom edges of the diaphragms, are in close proximity to these sector plates and minimize leakage of gas and air between sectors. In a typical rotary regenerative heat exchanger, the hot flue gas and the combustion air enter the rotor shell from opposite ends and pass in opposite directions over the heat exchange material housed within the rotor. Consequently, the cold air inlet and the cooled gas outlet are at one end of the heat exchanger, usually the bottom and referred to as the cold end, and the hot gas inlet and the heated air outlet are at the opposite end of the heat exchanger, usually the top and referred to as the hot end. As a result, an axial temperature gradient exists from the hot end of the rotor to the cold end of the rotor. In response to this temperature gradient, the rotor tends to distort and to assume a shape similar to that of an inverted dish (commonly referred to as rotor turndown). As a result, the radial seals mounted on the top edges of the diaphragms are pulled away from the sector plates with the greater separation occurring at the outer radius of the rotor. This opens a gap permitting the undesired intermingling of the gas and the air.
Various schemes have been developed to maintain the seal between the radial seals mounted on the diaphragms and the sector plates during this temperature change. It is well known that rotary regenerative air preheaters utilize a flexible sealing member that extends across the gap between the diaphragms and the sector plates. As the rotor transitions from a cold, non-operating condition to a hot, operating condition, the temperature gradient along the rotor increases, and the gap between the hot end diaphragms and the sector plates increases. These flexible sealing members are designed to maintain contact with the sector plate.
However, these seals are subject to a number of problems. It has been experienced that the continuous contact between the sealing member and the sector plates results in wear to both the sealing member and the sector plates. Special liners are sometimes utilized to reduce sealing surface wear. However, use of such liners results in higher capital and labor costs. Further, those seals are subject to premature failure due to flexure and edge fracturing. Finally, the amount of gap that may be closed by these seals is limited.